1. Field of the Invention
The present invention generally relates to photodetectors and, more particularly, to a photodetector that detects radiation within a spectral band ranging from terahertz to visible light, including infrared (IR) radiation. The disclosed detector comprises a high density nano-tunneling-junction-based photodetector array, rather than by conventional thermal or photodetectors. This detection mechanism improves response time and sensitivity, and avoids the need for cooling to cut thermal noise.
2. Description of Prior Art
A focal plane array (FPA) is a matrix of detector cells attached to a semiconductor chip. The detector cells absorb IR radiation, convert it into electrons, and send a voltage signal in response to form an image. Technically, FPAs operate much like a charge coupled device (CCD), which is a counterpart used in the visible light portion of the spectrum. CCDs are commonly used in cameras. The two main types of conventional photodetectors are thermal and photo detectors.
Thermal detectors sense the thermal effects of incident radiation in various ways. For example, bolometers and microbolometers sense changes in resistance. Thermocouples and thermopiles sense a thermoelectric effect. While these thermal detectors can be operated at room temperature, they present several drawbacks, particularly, low detectivity and slow response time, which limits their applications.
Photodetectors employ semiconductors in which incident photons cause electric excitations. An example is a photovoltaic detector based upon a p-n transistor junction, in which photoelectric current appears upon illumination. The response time and sensitivity of photodetectors can be much higher. Indeed, the current state-of-the-art IR photon detectors offers high detectivity (i.e. signal-to-noise ratio) and very fast response time. However, they require cryogenc cooling to suppress the thermally-generated noise in semiconductors. The requirement of cryo-cooling makes semiconductor-based photon detectors bulky, heavy, expensive, dependent on additional power supplies, and impractical for many applications.
Uncooled and room temperature sensors would offer unique advantages over existing cryogenic sensors with their complicated problems (including additional power requirements, thermal shielding, and limited lifetime) and additional weight and bulk, especially for space and airborne applications, provided that high detectivity and fast response times can be attained. “Uncooled” refers to those not cooled cryogenically to 77 K or below. “Room temperature” refers to those kept at above 300 K.
For example, infrared (IR) detectors that could operate without cryogenic cooling, and especially those that can operate at room temperature, have the potential to provide improved night vision capabilities for search and rescue applications as well as space and science applications, packaged in a device of extremely small size, weight, and power. This would significantly reduce the cost and accelerate the implementation of sensor for applications such as observation and surveillance, and also enable imaging sensors for new platforms such as robotics and micro-air vehicles.
Unfortunately, to date the performance of uncooled and room temperature sensors has been inferior to that of cooled sensors. This performance gap limits the number of applications and precludes the widespread use of uncooled infrared sensors in most photon (including IR) sensor applications. A survey of available sensors demonstrates this. For instance, uncooled thermal FPAs have been used in large array formats with an average f/1 noise-equivalent temperature-difference of 8.6 mK for VOx detectors and 30 mK for amorphous Si microbolometers. But thermal FPAs suffer from low detectivity (108-109 cmHz½/W) and slow response time (tens of msec).
On the other hand, high performance IR photon detectors, such as HgCdTe and InSb, require cryogenic cooling to suppress “dark current” in order to approach detectivities over 1011 cmHz½/W. The same issues arise with narrowband detectors like quantum-well-intersubband-photon detectors (QWIP).
Multi- or hyperspectral imagers deployed in NASA's Earth Observing System-1 (EOS-1) offer enabling capabilities for extracting IR signals through a plurality of environments. However, this and other known systems operating in a one-dimensional scanned mode are complex with moving optical filters, and often are power-hungry. Furthermore, current state-of-the-art FPAs are limited to no more than 4 colors, using either HgCdTe or quantum well infrared photodetectors (QWIP). The vertical integration of wavelength-selective QWIPs or HgCdTe detectors in a 3D cube configuration have an inherent limitation due to severe challenges in material growth, spectral resolution at each pixel, and read-out circuitry/power dissipation.
In sum, none of the current state-of-the-art imagers/FPAs offers the functionality of simultaneous multispectral and polarimetric sensing, and the conventional IR detection approaches that they rely on fail to achieve the necessary performance metrics. Consequently, what is desired is a hyperspectral and polarimetric photodetector configuration suitable for forming a focal plane array (FPA) with high resolution and high frame rate, but without any need for cryogenic cooling. This would have particular advantage for satellite-based imaging applications as well as a variety of other commercial applications.